A portion of the disclosure of this patent document contains material to which a claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction of any one of the patent documents or the patent disclosure, as it appears in the U. S. Patent and Trademark Office patent file or records, but reserves all other rights whatsoever.
Digitizer tablets are used to provide input signals to digital computers. A surface of the tablet is sensitized to the position of a stylus. The stylus is electrically or optically coupled to the digitizer tablet and causes a coordinate signal to be generated which signal corresponds to the coordinates of the point at which the stylus is positioned on the surface of the tablet. The generated coordinate signal is then used by the computer in real-time to select menu items from a keyboard selected and displayed menu. The stylus is generally activated by pressing it against the tablet surface. A paper overlay having defined function areas, or other file information relating to the keyboard selected menu, is positioned on the tablet surface and aligned with the X,Y coordinate axes of the tablet to facilitate the users positioning of the stylus on the tablet. The orientation of the overlay on the surface of the tablet and the scale of the overlay must be such that the position of the stylus on the overlay corresponds to the point on the tablet that will generate the correct coordinate signals for addressing the correct computer file. Misalignments will result in either addressing the wrong file, or no file at all. It would therefore be highly desirable to have a system that is not critical with respect to overlay size and position and which does not require a preidentification of the overlay description file. The present invention is such a system.